fetchToStore(): Fix caching

This was broken because MountedSourceAccessor did not return a
fingerprint. Previously fingerprints were global to an accessor, but
with a MountedSourceAccessor the fingerprint can be different for each
mount point.

src/libutil/include/nix/util/forwarding-source-accessor.hh

@@ -52,6 +52,16 @@ struct ForwardingSourceAccessor : SourceAccessor
     {
         return next->getPhysicalPath(path);
     }
+
+    std::optional<std::string> getFingerprint(const CanonPath & path) override
+    {
+        return next->getFingerprint(path);
+    }
+
+    void setFingerprint(std::string fingerprint) override
+    {
+        next->setFingerprint(std::move(fingerprint));
+    }
 };
 
 }

src/libutil/include/nix/util/source-accessor.hh

@@ -177,10 +177,27 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
         SymlinkResolution mode = SymlinkResolution::Full);
 
     /**
-     * A string that uniquely represents the contents of this
-     * accessor. This is used for caching lookups (see `fetchToStore()`).
+     * Return a string that uniquely represents the contents of this
+     * accessor. This is used for caching lookups (see
+     * `fetchToStore()`).
+     *
+     * Fingerprints are generally for the entire accessor, but this
+     * method takes a `path` argument to support accessors like
+     * `MountedSourceAccessor` that combine multiple underlying
+     * accessors. A fingerprint should only be returned if it uniquely
+     * represents everything under `path`.
      */
-    std::optional<std::string> fingerprint;
+    virtual std::optional<std::string> getFingerprint(const CanonPath & path)
+    {
+        return _fingerprint;
+    }
+
+    virtual void setFingerprint(std::string fingerprint)
+    {
+        _fingerprint = std::move(fingerprint);
+    }
+
+    std::optional<std::string> _fingerprint;
 
     /**
      * Return the maximum last-modified time of the files in this

src/libutil/mounted-source-accessor.cc

@@ -90,6 +90,15 @@ struct MountedSourceAccessorImpl : MountedSourceAccessor
         else
             return nullptr;
     }
+
+    std::optional<std::string> getFingerprint(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        // FIXME: check that there are no mounts underneath the mount
+        // point of `accessor`, since that would invalidate the
+        // fingerprint. (However we don't have such at the moment.)
+        return accessor->getFingerprint(subpath);
+    }
 };
 
 ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)